1. Field of the Invention
This invention relates to the in-situ gasification of a subterraneous coal deposit for the recovery of a product gas having a predetermined H.sub.2 /CO ratio. More particularly, the present invention is a method for producing a product gas by in-situ gasification of coal wherein carbon dioxide recovered from the product gas is recycled into the coal deposit with steam and oxygen to control the H.sub.2 /CO ratio of the product gas to a predetermined valve.
2. Background of the Invention
In-situ gasification of a subterranean coal deposit is a useful method for the in-situ extraction of coal value. In its most general form, a pair of process wells (an injector and a producer) some specified distance apart, is drilled from the surface into the coal deposit. Although a coal deposit contains an appreciable amount of natural cracks and fissures, its overall permeability is quite low. Therefore, prior to initiation of in-situ gasification, fluid communication passages must be established in the coal deposit between the two wells. Fluid communication passages may be established by various means such as directional drilling, hydraulic, or explosive fracturing, reverse combustion, etc. Thereafter, a mixture of steam and an oxidant such as air, oxygen-enriched air or essentially pure oxygen is introduced into the coal deposit via the injection well to initiate an oxidation reaction that forms a product gas containing carbon monoxide, hydrogen, and carbon dioxide that is recovered through the production well.
The product gas recovered in which hydrogen and carbon monoxide are the principal ingredients is useful as a feed stock for various Fischer-Tropsche synthesis processes. In these processes, the H.sub.2 /CO ratio of the feed gas is critical to the efficiency of the particular process. For example, in the conversion of synthesis gas comprising hydrogen and carbon monoxide to methanol, the optimum H.sub.2 /CO ratio is 2.0.
The present invention provides a method by which the H.sub.2 /CO ratio of the product gas produced by in-situ coal gasification can be controlled.